1. Field of the Invention
This invention applies to micro-lithography and the application of micro-lithography as a tool in the development and production of micro-electronic and micro-mechanical devices and of integrated circuits.
2. Description of Prior Art
Background.
Ultra High Resolution Lithography has many advantages but one significant shortcoming when it is used for printing asymmetric, two-dimensional patterns.
The printing is achieved by demagnifying clear mask features without the use of either lenses or mirrors between a mask and a resist. The resist is coated onto a wafer and the mask is placed in proximity to the wafer, separated by a precise gap. The demagnification results from the positive use of two-sided bias in Ultra High Resolution Lithography. Typically, X-rays are used for exposing the resists, spin-coated onto a silicon wafer, and placed near a Critical Condition with respect to the mask. Besides this mask-wafer gap, the Critical Condition depends on clear mask feature size and on the wavelengths of radiation used.
In Ultra High Resolution Lithography, as in Next Generation Lithography, the classical concept of fidelity in the reproduction of masks had been relaxed. The use of masks with comparatively large clear features and used with comparatively large mask-wafer gaps provided unexpected extensibility to Proximity X-ray Lithography, which had previously required classical fidelity in the reproduction of masks, including 1:1 printing (i.e. not demagnified). The extensibility is accompanied by many further subsidiary benefits including the elimination of well-known side-bands often observed previously in the printing of periodic structures. The fact that neither high precision lenses nor high precision mirrors are needed and that the light source is typically bright, with short exposure times and high throughput, provides significant advantages for the production of next generation semiconductors.
Among NGLs competing for sub-100 nm patterning, Proximity X-ray Lithography is the most advanced and mature, so that extensibility, through Ultra High Resolution Lithography, is of special significance. The technique can be used as much for the printing of modem semiconductor integrated circuits as for fabricating micromachines and micro-electro-mechanical systems. However, printing from asymmetrical two dimensional masks produces special effects which are corrected in the present invention.
Ultra High Resolution Lithography has been demonstrated to produce prints at 25 nm spacing from one-dimensional line grids. If patterns to be printed are not symmetrical, intensity variations occur along different axes. This is chiefly because the Critical Condition cannot then be maintained accurately for both (e.g. vertical and horizontal) axes. The present invention provides a procedure for optimizing Ultra High Resolution Lithography when printing asymmetric two-dimensional patterns. The invention shows how to correct the variations, by applying temporal and spatial coherence in wave interference at the Critical Condition, and through the use of adjustments to the shapes of masks. The invention has special significance for applications using rapid exposures with broad band radiation sources, since, with these sources, temporal coherence is otherwise generally detrimental to resolution unless properly managed.
The invention is a unique method that is specific to ultra high resolution lithography. The method is used to define mask shapes which differ from all previous methods for proximity corrections because this method alone uses temporal and spatial coherence, with broad band sources, near the Critical Condition and without the use of lenses or mirrors between mask and wafer. The method requires distinctive mask shapes. The method is also uniquely adapted to the printing, with 1 nm wavelength X-rays, of features with dimensions about 20 nm. In these features the invention differs from those of:
U.S. Pat. No. 6,383,697, to Vladimirsky et al. “Ultra High Resolution Lithographic Imaging and Printing and Defect Reduction by Exposure near the Critical Condition.
U.S. Pat. No. 6,194,104 to Hsu that describes a method for improving a lithography process window by employing scaler functions with normal and area vectors.
A paper by O. W. Otto et al. “Automated optical proximity correction—a rules-based approach,” Optical/Laser Microlithography VII, Proc. SPIE (2197) 1994, pages 278-293 describes a rules based approach for optical proximity correction.
A paper by S. Shioiri and H. Tanabe “Fast Optical Proximity Correction: Analytical Method,” Optical/Laser Microlithography VIII, Proc. SPIE (2440) 1995, pages 261-269 describes a method for calculating proximity corrected features analytically.